1. Field of the Invention
The present invention concerns a method for image generation by magnetic resonance, of the type wherein a spectral component of the magnetic resonance signal should be suppressed in the signal acquisition.
2. Description of the Prior Art
In the acquisition of MR images the human body, due to their concentration only the hydrogen nuclei of free water and in fat bonds respectively contribute to the MR signal. Their relative resonance frequency difference is approximately 3.4 ppm (parts per million). In many applications in MR imaging, it is desirable to suppress the fat signal contribution or signal contributions of other spin portions as well. For example, in order to suppress the interfering influence of the fat signal contribution it is known to suppress the spectral component of fat by what is known as a fat saturation or an inversion technique. In this technique a frequency-selective radio-frequency pulse (RF pulse) is used in order to saturate or to invert the fat signal portions before the image generation is begun with the actual signal acquisition. Such fat suppression pulses are applied each time before the actual MR imaging. In the case of fast imaging sequences this can mean that such fat saturation pulses are radiated into the examined body in a relatively fast sequence, for example within a few tens of milliseconds, with the point in time until the repetition of the next fat saturation pulse being on the order of, or smaller than, the T1 relaxation time of the fat signals. This leads to an equilibrium state of the fat signal portions. The application of a few RF pulses is typically required in order to reach this equilibrium state. Strong fluctuations in the fat magnetization can occur before this equilibrium state is reached. This means that, for example, the fat signals are not precisely inverted by 180° given a 180° inversion pulse.
At high basic magnetic field strengths of B1, the problem furthermore exists that the field strength of the radiated radio-frequency field for saturation of the fat signal portions is very inhomogeneous. This leads to the situation that the signal portions to be inverted experience different flip angles. This has the result that, in the signal acquisition for image generation, the fat signal contributions still undesirably contribute with varying strength to the detected signal, and thus to the signal intensity.
For a three-dimensional gradient echo sequence, the optimal point in time at which the fat suppression pulse should be radiated can be calculated using the sequence parameters such as repetition time, flip angle of the fat suppression, flip angle of the gradient echo sequence, etc. (see also Gunar Brix et al. in “Fast and Precise T1 Imaging Using a Tomrop Sequence” in Magnetic Resonance Imaging, Vol. 8, pages 351-356, 1990). However, this point in time TI, that defines the point in time of the RF pulse before the actual MR signal acquisition ensues, depends strongly on the excitation angle of the 3D gradient echo sequence. This optimal point in time TI for the fat suppression is shown in FIG. 1 as a function of the excitation flip angle. FIG. 1 shows the change of the point in time TI for various excitation angles of the water protons in the actual imaging. As can be seen in FIG. 1, the point in time TI varies strongly for various angles. A long TI time is required for small flip angles while the TI time should be shortened for larger flip angles in order to obtain an optimal fat suppression.
Since, as mentioned above, the radiated B1 field for excitation can be inhomogeneous at high basic magnetic field strengths B0, this leads to an inhomogeneous fat suppression. Even if a type of pulse known as a non-B1-sensitive adiabatic RF pulse is used for the fat suppression, this leads to an inhomogeneous fat suppression since the non-sensitive B1 field excitation is not possible in the actual imaging sequence. This leads to the situation that the optimal point in time for fat suppression differs for different regions of the examined tissue. The use of adiabatic excitation pulses in the imaging sequence itself would lead to unacceptable SAR (specific absorption rate) values, such that too much power would be radiated into the examination region. Furthermore, an adiabatic excitation pulse would dramatically extend the temporal length of the excitation pulse, which would have a negative effect on the acquisition time.